DOCS-685 add powersof2 sentence from Dan

Author: kay-kim

source/core/write-operations.txt

@@ -364,8 +364,10 @@ padding for new inserts and moves.
 .. versionadded:: 2.2
    You can use the :dbcommand:`collMod` command with the
    :collflag:`usePowerOf2Sizes` flag so that MongoDB allocates document
-   space in sizes that are powers of 2. As with all padding, using
-   powers of 2 minimizes, but does not eliminate, document movements.
+   space in sizes that are powers of 2. This helps ensure that the
+   space freed due to either deletions or document relocations is
+   reused efficiently. As with all padding, using powers of 2
+   minimizes, but does not eliminate, document movements.
 
 To check the current :stats:`paddingFactor` on a collection, you can
 run the :dbcommand:`db.collection.stats()` command in the

source/faq/developers.txt

@@ -678,7 +678,9 @@ grow on subsequent writes. You can change the default
 :stats:`paddingFactor` calculation by using the the
 :dbcommand:`collMod` command with the :collflag:`usePowerOf2Sizes`
 flag. The :collflag:`usePowerOf2Sizes` flag ensures that MongoDB
-allocates document space in sizes that are powers of 2.
+allocates document space in sizes that are powers of 2, which helps
+ensure that the space freed due to either deletions or document
+relocations is reused efficiently.
 
 However, in those exceptions where you must pad manually, you can use
 the strategy of first adding a temporary field to a document and then